<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <script src="http://libs.baidu.com/jquery/1.9.0/jquery.js"></script>
</head>
<body>
<li>aaaa</li>
<li>bbbb</li>
<li>cccc</li>
<script>
    $('li').css('color','red').addClass('nav')
</script>
</body>
</html>